1. Field of the Invention
The present invention relates to a pellicle for photolithography, and particularly relates to a pellicle for photolithography used as dust-proof protection in manufacturing of semiconductor devices such as LSIs and ULSIs, or liquid crystal display panels. More particularly the present invention relates to a pellicle for photolithography used in ultraviolet exposure of 200 nm or shorter, which is employed in exposures required to have high resolution.
2. Description of the Related Art
Conventionally, manufacturing of semiconductor devices such as LSIs and ULSIs, or liquid crystal display panels, and such, involves patterning a semiconductor wafer or liquid crystal plate by irradiating light thereon. However, this is problematic in that if any dust adheres to the photomask used in such cases, this dust absorbs or reflects light, altering, and roughening the edges of the transferred pattern, thereby compromising the dimensions, quality, appearance, and reducing the performance and manufacturing yield of the semiconductor devices, the liquid crystal display panels, or such.
For this reason, these operations are usually carried out in a clean room. However, since even in a clean room it is difficult to always keep an photomask clean, in order to protect the photomask from dust, a pellicle which is substantially transparent to the exposure light and which protects from dust, is attached to the surface of the photomask.
This is advantageous, as dust does not adhere directly onto the surface of the exposure plate, but onto the pellicle membrane, and therefore, dust on the pellicle membrane will not effect pattern transfer provided that focus is fixed on the pattern of the photomask at the time of photolithography.
This kind of pellicle is formed from, a transparent pellicle film formed from a significantly transmissive nitrocellulose, cellulose acetate or such, which is bonded onto the upper surface of a pellicle frame formed from aluminum, stainless steel, polyethylene or such by means of a good solvent of the pellicle film which is coated on the upper surface of the pellicle frame and air dried (for example, see Japanese Patent Application Laid-open (kokai) No. 58-219023); or alternatively by means of an adhesive, such as epoxy resin (for example, see U.S. Pat. No. 4,861,402 specification or Japanese Patent publication (kokoku) No. 63-27707), acrylic resin or fluoropolymer adhesive (for example, see Japanese Patent Application Laid-open (kokai) No. H7-168345). Furthermore, a pressure-sensitive adhesive layer formed from polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin or such, and a release layer (separator) for protecting the pressure-sensitive adhesive layer, are both bonded to the underside of the pellicle frame.
In Recent years, resolution required in photolithography is increasing. In order to realize these increased resolution, light having shorter wavelength is gradually being used as the light sources. More specifically, there is a shift towards ultraviolet light [g-line (436 nm), i-line (365 nm), KrF excimer laser (248 nm)], and recently ArF excimer laser (193 nm) are beginning to be used.
In a semiconductor exposure system, a pattern formed on a photomask is transferred onto a silicon wafer by light having a short wavelength. However, if the surfaces of the photomask (hereinafter briefly referred to as “mask”) and the wafer have irregularities, the irradiating light becomes unfocused at the time of exposure, causing problems to arise in the pattern to be transferred. As the refinement of semiconductors proceeds forward, the flatness demanded for the mask and silicon wafer is gradually becoming more and more exacting. For this reason, the flatness demanded for the pattern surface in masks is also gradually becoming more exacting with demands from 2 μm, to 0.5 μm and 0.25 μm for nodes of smaller than 65 nm.
A pellicle is attached to the mask to protect the pattern from dust after the mask is completed, however, when the pellicle is attached to the mask, the flatness of the mask is altered. This phenomenon is considered to be caused by effects of the pellicle frame irregularity, on the flatness of the mask.
The pellicle is attached to the mask by means of the pressure-sensitive adhesive on one side of the pellicle frame. However, when the pellicle is attached to the mask, conventionally a force of about 20 to 30 kg is used to press and attach the pellicle to the mask. Generally, the flatness of the mask in TIR values is several μm or less, with the latest masks having flatness of 1 μm or less, however, the flatness of the pellicle frame in TIR values is generally large when compared with the mask and of several dozen μm. As a result, when the pellicle is attached to the mask, the flatness of the mask is altered due to the irregularity of the pellicle frame. Thus, it is conceivable that if the flatness of the pellicle frame is raised to the level of flatness of the mask it would be possible to reduce the change in the flatness of the mask.
The pellicle frame is usually made from an aluminum alloy. A pellicle frame for semiconductor photolithography is about 150 mm in width and about 110 to 130 mm in length with the middle portion removed. Generally, the pellicle frame is made by cutting the pellicle frame shape from a plate of aluminum alloy or, extrusion molding aluminum material in a frame shape. However, making a flat frame is not easy as the width of the frame is thin and about 2 mm and thus easily deformed. Hence, achieving flatness of the pellicle frame at the level of flatness of the mask, is extremely difficult.
In view of the above circumstances, the object of the present invention is to realize a smoothness in a mask surface to which a pellicle is attached, wherein the smoothness is not dependent on the smoothness of one end surface of a pellicle frame.